Borole Derivatives. 22. Iodine Degradation of the Triple-Decker Complexes (μ-C4H4BR)[Rh(C4H4BR)]2 (R = Ph, Me):  Synthesis and Structural Characterization of Heterocubanes [Rh(μ3-I)(C4H4BR)]4 and of Bis(borole)rhodium Complexes RhI(C4H4BR)2 1

The triple-decker complexes (μ-C4H4BR)[Rh(C4H4BR)]2 (R = Ph, Me) (1) react with elemental iodine in toluene or dichloromethane to produce heterocubanes [Rh(μ3-I)(C4H4BR)]4 (2) and bis(borole)iodorhodium complexes RhI(C4H4BR)2 (3). The heterocubanes 2 exchange fragments RhI(C4H4BR) at rates which are fast with respect to the NMR time scale. The bis(borole) compounds 3 exist in an equilibrium of rotational isomers C s -3 = C 2 v -3 with ΔR H = (−1.7 ± 0.2) kJ mol-1 and ΔR S = (−16 ± 1) J mol-1 K-1 for 3b. Activation parameters for the forward and reverse isomerization reactions are reported. The isomerization takes place by the mechanism of ring−ring rotation; this is demonstrated for 3b by TOCSY NMR spectra (1H NMR, −80 °C, mixing time 150 ms). Crystal structure determinations of 2a, 2b, and 3b are reported. The higher-energy isomer C s -3b is found in the crystal of 3b. One borole ligand of C s -3b is planar and η5-bonded with a normal Rh−B1 bond distance [236.7(6) pm]. The second ligand shows an unusual folding of 15.8(9)° along the line C2−C5, a very weak bonding interaction Rh−B2 [252.8(6) pm] approaching η4-coordination, and a close contact I−B2 [295.3(6) pm]. Both borole ligands show diene-like C−C bond length patterns with weak Rh-to-borole back-bonding.